The present application relates to the art of metal processing. The invention finds particular application in conjunction with cooling strip steel after immersion in a galvanizing bath and in preparation for minimizing the spangle size of the steel coating. This invention will be described with particular reference thereto. It is to be appreciated, however, that the invention is also applicable to other strip handling systems in which selective cooling across the strip width is advantageous.
Conventionally in preparation for minispangling, hot strip steel is dipped into a bath of molten zinc. The excess zinc is wiped off the steel by a coating weight control device, and then the steel and coating are allowed to cool naturally in air or are uniformly cooled by forced air coolers. In many cases the strip steel and coating are not a uniform temperature across the width of the strip. This non-uniform temperature is a result of many factors such as variations in steel strip and coating thicknesses, the natural tendency of the strip edges to cool faster than the center of the strip, non-uniform cooling by the coating weight control device and non-uniformities in thermal treatments of the strip prior to the galvanizing bath. When the steel strip and coating temperature are not uniform across the strip prior to entry into the minispangle box, the minispangle coating appearance will be non-uniform. In this case, one area of strip may have minimized spangles and another area will have partially reduced or full size spangles (irregular shaped crystals) as seen on regular galvanized steel strip.
The present invention contemplates a new and improved differential pre-cooling apparatus which selectively provides non-uniform cooling across the strip width to compensate for inherent uneven steel strip and coating temperatures.